Interdigital magnetron



Aug. 21, 1951 H. J. M CARTHY INTERDIGITAL MAGNEITRON 2 SheetsSheet 1Filed Oct. 51, 1946 a INVENTOR. ZFury JLJIIWLHIIhy 54W raw ATTORNEY Aug.21, 1951 H. J. MCCARTHY INTERDIGITAL MAGNEITRON 2 Sheets-Sheet 2 FiledOct. 51, 1946 R. W Y N E 5 N Vr R m a m T 9 A W J U rw H v a 0 5 R E T|E M .H N m 6.! T .7 G M1 E IL W A W PDnFDO kwBom mafia Gm Patented Aug.21, 1951 INTERDIGITAL MAGNETRON Henry J. McCarthy, Danvers, Mass.,assignor to Sylvania Electric Products Inc., Salem, Mass.,. acorporation of Massachusetts 7 Application October 31, 1946, Serial No.706,861

13 Claims.

This invention relates to microwave devices and particularly tomicrowave generators such as interdigital magnetrons and the like.

'of a cylinder or the like, the sets being intermeshed but not incontact, with the circuit between the two sets of teeth completedthrough a resonant cavity.

In such an arrangement there are radial modes of oscillation andcircumferential or angular modes. In the latter, the field strengthvaries around the cylinder formed by the teeth, and the coupling is bestwhen the coupling loop is placed in the plane of the voltage maximum.However, as the coupling is increased, or the anode voltage raised, thevoltage maxima tend to shift away from the loop, and may eventuallyshift 90, under which condition the device may oscillate strongly withno power output to the coupling loop. This mode of operation isdescribed in copending applications Serial No. 619,289, filed September29, 1945, by Donald L. Benedict, and Serial No. 719,414, filed December31, 1946, by Robert M. Bowie.

I have found that this may be prevented by short-circuiting the two setsof teeth at the voltage nodes, preferably by making the teeth at thenodes longer than the other teeth and long enough to touch the ringwhich usually connects the other set of teeth. This allows oscillationsonly for those modes at which a voltage node occurs at-theshort-circuited teeth. The angular mode of oscillation can no longershift around the toothed cylinder, and the radial mode isshort-circuited.

Diametrically opposite teeth may be shortcircuited for the fundamentalangular mode, and additional teeth may be short-circuited if harmoniesare preferred. Shorted teeth have increased the power output five tlmesin some cases.

The further objects, advantages and features of the invention will beapparent from the following specification, taken in conjunction with theaccompanying drawings.

Figure 1 is a partially cut-away View of a device according to theinvention;

Figure 2 is a developed view of the opposing anode teeth showing theshort-circuiting teeth .and the variation of voltage and current alongposition of the coupling loop with respect to the teeth; and

Figure 4 is a curve of power output against wavelength as the distancebetween the teeth is varied.

In Figure 1, the evacuated glass envelope 1 has sealed therethrough thecathode lead-in wires 2 and 3 and the annular metal discs 4 and 5.Leadin wire 3 is bent to support the Wire 6 along the axis of theenvelope I. The cathode sleeve 1 is concentric with and supported fromthe wire 3 by the washers 8, 9 which close the ends of the cathodesleeve 1. Connectingstraps I0, H hold sleeve 1 to the wire 6 and insurefirm electrical connection therewith. The cathode heater coil I2surrounds the wire 6 but is spaced or electrically insulated therefromto prevent shorting. One end l3 of heater coil I2 is connected tolead-in wire 2, the other end I5 to support wire It welded T-like towire 6, the back bone of the cathode structure as a whole. Sleeve 1 maybe of nickel, preferably highly purified, and may be coated with anelectron emitting surface l! preferably of barium, strontium and calciumoxides. Guard washers [B and I9 may be used at each end of the emittingsurface I1, but will be spaced from the anodes 20, 2|. These anodes 29,2| comprise rings 22, 23 transverse to the longitudinal axis of thecathode with the fingers or teeth 24, 25 extending parallel to thecathode sleeve 1.

The fingers, such as 24 extending downward from the top ring 22 mesh orinterleave with those extending upward from the bottom ring 23 but donot touch them except for the two diagonally opposite fingers 253C and26 which are in electrical contact with the opposing rings. Theremaining fingers 2'! to 49 inclusive, do not contact their opposingrings. This is shown in Figure 2, where the ring is developed.

Rings 22 and 23 are firmly connected electrically and. mechanically to,respectively, annular discs 4 and 5, which extend through the glassenvelope I, being sealed thereto in the usual manner well-known in theart. The lower ring 5 has attached to itself a flanged screw-threadedmetal cylinder 4| to facilitate placing the tube in a suitable cavity orsocket,

Theannular discs 4 and 5 are connected together externally by the metalpiece 50 to form a resonant cavity. The coupling loop 42 should be in aplane perpendicular to the plane of the shorted teeth 25:: and 26, andin a plane passing through the axis of the cylinder formed by the teeth.

A magnet 5|, is used; to produce a longitudinal field through the tube.

. sleeve 1 to the anodes 20, 2|.

The sealed exhaust tip 44 is merely the tubulation from which theenvelope or tube was exhausted. The longitudinal wires 45 and 46 aflixedto the transverse wire 41, in turn affixed to the backbone wire 6, aidin supporting the cathode sleeve 1.

In operation, a suitable heater voltage is applied to the lead-in wires2 and 3, to raise the cathode sleeve 1 and its coating I1 to atemperature sufficient for copious electron emission, and a voltageapplied between lead-in wire 3 and the anodes to cause electrons to bedirected from The electron motion due to this voltage will be radial. Amagnetic field is then applied along the axis of the tube and in thespace between the cathode sleeve 1 and the anodes 20, 21. This deflectsthe electrons in a direction normal to their radial path, causing aprecession of electrons before the teeth 25 to 53, inclusive, in theusual magnetron manner, causing oscillations in the anode circuit whenthe voltage and field move the electrons at the proper speed to excitesome mode of the anode structure.

In the first mode, sometimes called the radial mode, the field strengthvaries radially along the cavity, and is constant angularly. In thismode, the cavity has considerable influence on the frequency. With thecontacting teeth in the structure shown, this mode is short circuited.

The contacting teeth do not, however, shortcircuit the angular mode,because in this mode the oscillations follow the zig-Zag path betweenteeth (the cavity having a little effect except as a coupling means),and the voltage and current distributions around the ring are as shownin Figure 2, where the ring is shown in developed form. The shortedteeth are at the voltage nodes 48, 49.

Figure 6 is for a tube having two opposite teeth 0.005 longer than theothers, and shows the power output against wavelengths of oscillation,the wavelengths being changed by moving the teeth closer together untilthey contact. The solid line is for a coupling loop perpendicular to theplane of the teeth, and the dotted lines for a loop in that plane. Whenthe longer teeth meet, the power in the latter loop drops sharply tozero, while the power in the other loop rises from 1.8 to 8.5 watts. Ingetting the data for this curve one set of teeth was mounted on aflexible diaphragm which could be moved back and forth at its centerfrom outside the tube until the longer teeth were shorted to the otherpart of the anode.

If the wavelength variation possible with such a diaphragm is desired,and the advantage of the shorted teeth desired at all wavelengths, aflexible short-circuiting tooth is necessary. For example, the shortingtooth, projecting from one ring, may slide within a split-cylindercontacting it and attached to the other ring.

What I claim is:

1. In an interdigital magnetron, an anode comprising two oppositelyplaced sets of teeth, each set coaxial with the other and each setcomprising a series of electrically connected spaced teeth arrangedlongitudinally along the surface of an imaginary cylinder, the teeth ofthe two sets being in contact at least at one of the voltage nodes of apredetermined mode of oscillation, the remaining teeth of the two setsbeing spaced from 'each other.

2. In an interdigital magnetron, an anode comprising two oppositelyplaced sets of teeth, each set coaxial with the other and each setcomprising a series of electrically connected spaced teeth arrangedlongitudinally along the surface of an imaginary cylinder, the teeth ofthe sets being in contact only at two diagrammatically opposite points.

3. A resonator circuit comprising two oppositely placed sets of teeth,each set coaxial with the other and each set comprising a series ofelectrically connected spaced teeth arranged longitudinally along thesurface of an imaginary cylinder, the teeth of the two sets being incontact at least at one of the voltage nodes of a predetermined mode ofoscillation, the remaining teeth of the two sets beingspaced from eachother.

i. A resonator circuit comprising two oppositely placed sets of teeth,each set coaxial with the other and each set comprising circularconnecting means and a series of electrically connected spaced teetharranged longitudinally along the surface of an imaginary cylinder, themajority of the teeth of each set being separated from the other set ofelectrically connected teeth, said sets embodying isolated diametricallyopposite conductive portions interconnecting the two sets of teeth andtheir circular connecting means.

5. A microwave generator comprising an insulating envelope, acylindrical cathode within said envelope, a first conducting ringconcentric with said cathode and having spaced teeth extending normallyto the plane. of said first ring, a second conducting ring concentricwith said cathode and having spaced teeth extending normally to theplane of said second ring, said second and first rings being coaxial sothat their teeth intermesh along an imaginary cylindrical surface, theteeth of the two sets being in contact at least at one of the voltagenodes of a predetermined mode of oscillation, the remaining teeth of thetwo sets being spaced from each other.

6. A microwave generator comprising an insulating envelope, acylindrical cathode within said envelope, a first conducting ringconcentric with said cathode and having spaced teeth extending normallyto the plane of said first ring, a second conducting ring concentricwith said cathode and having spaced teeth extending normally to theplane of said second ring, said sec- .ond and first rings being coaxialso that their teeth intermesh along an imaginary cylindrical surface,the teeth of the sets being in contact only at two diametricallyopposite points.

7. The combination of claim 5, including a pair of spaced annularconducting discs, each of said discs being affixed to one of said rings,each of said discs extending through said envelope, a metalpath-connecting said discs outside said envelope to form a resonatorcavity, and a coupling loop in said cavity and perpendicular to theplane 'of said diametrically opposite teeth.

'8. In an interdigital magnetron, an anode comprising two oppositelyplaced sets of teeth, each set coaxial with the other and each setcomprising a series of electrically connected spaced teeth arrangedlongitudinally along the surface of an imaginarycylinder, certain of theteeth of the two sets interconnecting the sets of teeth at least at oneof the voltage nodes of a predetermined mode of oscillation, theremaining teeth of the two sets being spaced from each other, a resonantcavity encircling and interconnecting said sets of teeth, and a couplingloop extending into said cavity in a radial plane perpendicular to the.ra'dialrplane containing said interconnecting teeth.

9. In an interdigital magnetron, an anode comprising two oppositelyplaced sets of teeth, each set coaxial with the other and each setcomprising a series of electrically connected spaced teeth arrangedlongitudinally along the surface of an imaginary cylinder, the teeth ofone set being in contact with teeth of the other set only at twodiametrically opposite points, a resonant cavity encircling andinterconnecting said sets of teeth, and a coupling loop projecting intosaid resonant cavity in a radial plane perpendicular to the plane ofsaid diametrically opposite contacting teeth.

10. An interdigital magnetron comprising an insulating envelope, acylindrical cathode within said envelope, a first conducting ringconcentrically about said cathode near one end thereof, a secondconducting ring concentrically about said cathode near the opposite endthereof, teeth extending from said first and second rings respectivelyalong the surface of an imaginary cylinder, the teeth extending fromeach ring being interposed between the teeth extending from the otherring, and discs fixed to each of said rings and sealed through the wallof said envelope so as to afford external circumferential terminals forsaid rings, the teeth of each ring being separated from the teeth of theopposite ring except for certain teeth of each ring at voltage nodalpoints which interconnect said rings.

11. An interdigital magnetron including a cylindrical cathode, an anodeassembly including a ring concentrically around each end of saidcathode, each ring having teeth extending along the surface of animaginary cylinder and between the teeth of the opposite ring, thefingers of each ring being separated from the opposite ring except for aminor proportion of teeth extending from one ring to the other toestablish voltage nodal points.

12. An interdigital magnetron including a cathode and an anode assemblyof opposite anode members including two sets of teeth disposed along thesurface of an imaginary cylinder substantially coaxially about saidcathode, the fingers of each set having an interconnecting ring andbeing interposed between the fingers of the opposite anode member, thefingers of each anode member being separated from the opposite anodemember except for a minor proportion of teeth extending from one ring tothe other for the purpose of fixing a voltage node.

13. A high frequency device including two sets of fingers disposed in animaginary cylindrical surface, a resonant cavity encircling saidfingers, and embodying a pair of axially spaced rings, the fingers ofeach set extending between the fingers of the other set and each beingconnected to only one of said rings except for a minor proportion offingers which extend from one ring to the other to establish nodalpoints.

HENRY J. MCCARTHY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,147,159 Gutton et a1 Feb. 14,1939 2,247,077 Blewett et al June 24, 1941 2,250,698 Berline July 29,1941 2,409,222 Morton Oct, 15, 1946 2,417,789 Spencer Mar. 18, 19472,419,172 Smith Apr. 15, 1947 2,504,970 Engelmann Apr. 25, 19502,508,280 Ludi May 16, 1950

